That's great! They don't advertise this, unfortunately, at plantuml.com :/

On Tue, 2 Jan 2018 at 13:57 Yahya Abou 'Imran <yahya-abou-imran@protonmail.com> wrote:
And on the sourceforge page:

https://sourceforge.net/projects/plantuml/

License
GNU General Public License version 2.0 (GPLv2)